JWST Planetary Observations within the Solar System

Jonathan Lunine

Department of Physics

The University of Rome, Tor Vergata

Heidi Hammel

Space Science Institute, Boulder, CO

Emily Schaller

Lunar and Planetary Laboratory

The University of Arizona

George Sonneborn

NASA Goddard Space Flight Center

Glenn Orton

NASA Jet Propulsion Laboratory

California Institute of Technology

George Rieke

Steward Observatory, The University of Arizona

Marcia Rieke

Steward Observatory, The University of Arizona

March 9,2010

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https://ntrs.nasa.gov/search.jsp?R=20100015397 2020-06-11T21:43:52+00:00Z

Abstract

JWST provides capabilities unmatched by other telescopic facilities in the near to mid

infrared part of the electromagnetic spectrum. Its combination of broad wavelength

range, high sensitivity and near diffraction-limited imaging around two microns

wavelength make it a high value facility for a variety of Solar System targets. Beyond

Neptune, a class of cold, large bodies that include Pluto, Triton and Eris exhibits surface

deposits of nitrogen, methane, and other molecules that are poorly observed from the

ground, but for which JWST might provide spectral mapping at high sensitivity and

spatial resolution difficult to match with the current generation of ground-based

observatories. The observatory will also provide unique sensitivity in a variety of near

and mid infrared windows for observing relatively deep into the atmospheres of Uranus

and Neptune, searching there for minor species. It will examine the Jovian aurora in a

wavelength regime where the background atmosphere is dark. Special provision of a sub­

array observing strategy may allow observation of Jupiter and Saturn over a larger

wavelength range despite their large surface brightnesses, allowing for detailed

observation of transient phenomena including large scale storms and impact-generation

disturbances. JWST's observations of Saturn's moon Titan will overlap with and go

beyond the 2017 end-of-mission for Cassini, providing an important extension to the

time-series of meteorological studies for much of northern hemisphere summer. It will

overlap with a number of other planetary missions to targets for which JWST can make

unique types of observations. JWST provides a platform for linking solar system and

extrasolar planet studies through its unique observational capabilities in both arenas.

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Introduction

The James Webb Space Telescope continues the legacy of Hubble Space Telescope in

observing solar system objects with the unique wavelength coverage and image quality

available to space-borne observatories (Gardner et aI., 2006). JWST brings much higher

sensitivity and wavelength range to observing such targets. The moving target capability

being implemented for the observatory will enable tracking and study of a wide range of

interesting solar system objects (Balzano et aI., 2008). JWST also will have capabilities

for observing exoplanets that extend beyond what can be done from the Earth, and

particularly for M dwarfs will permit characterization of exoplanets down to sizes a few

times that of the Earth. In this white paper we highlight selected examples of solar

system observations which JWST can do, in some cases uniquely, by virtue of its high

sensitivity and broad wavelength coverage. Many of these objects are very bright, and

special techniques to enable their observation are detailed in a separate report by Meixner

et al. (2008). A separate white paper is being prepared by Clampin et al. on extrasolar

planet observations, and hence is not covered here. The material presented here is derived

largely from a workshop conducted at the 40th DPS meeting in Ithaca New York, on

October 10, 2008.

In this paper we do not cover the inferior planets, simply because JWST is unable

to observe them. Because of its architecture, especially the size and shape of the

sunshield, sight lines from JWST may be no closer than 85 degrees from a line from the

sun to the observatory axis (figure 1).

The importance of studying the solar system

With over 420 planets and 41 multiple planet systems in the nearby regions of the

Galaxy, our solar system has become a kind of ground truth for what is a common cosmic

phenomenon. Study of the planets of our solar system provides detailed information on

atmospheric and surface processes that has already provided dividends in interpreting

observations of extrasolar planets. For example, fluid dynamical models of atmospheric

circulation in extrasolar planets derive from models constrained by the dynamical

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phenomena observed on the solar system's giant planets (Showman et al. 2010).

Observations of the terrestrial planets with atmospheres and of Titan, whose dense

nitrogen-dominant atmosphere is part of an active hydrologic cycle involving methane,

provide information on the range of potential terrestrial-type planets that will be observed

around Sun-like stars and M-dwarfs. Studies of the most distant and primitive bodies in

the solar system, including comets and Kuiper Belt objects, provide clues to the original

inventory of material from which the solar system formed, and this information can be

compared with and placed in the context of compositional studies of molecular clouds

and the interstellar medium to better constrain the cycle of matter in the galaxy (Gardner

et aI., 2006) (figure 2). Finally, selected bodies may reveal spectroscopic features

potentially indicative of biological or interesting prebiotic organic processes, so that

telescopic observations become remote sensing searches for life.

Exclusion zone <45 from Anti-Sun I

Exclusion zone 85" from Sun

Sun

Figure 1. Allowable field-of-regard for JWST, showing the exclusion zones in the solar and anti solar directions. From Gardner et al (2006).

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Figure 2. Comets and circumstellar disks. An ISO-SWS spectrum (speed 4, corresponding to a spectral resoution of approximately 1000) of the Herbig AelBe star HD 100546 (upper curve) shows emission features in the star's circurnstellar disk as predicted if the disk is heated by radiation from the central source. The particle composition in the circurnstellar disk appears to be remarkably similar to that of comet Hale-Bopp (lower curve). The wavelength range shown here corresponds to that which will be covered by JWST. (Figure modified from Malfait et al., 1998).

Selected solar system objects

In this paper we move outward from the Earth to the edge of the solar system and

examine selected objects and classes of objects for which JWST can make useful

scientific contributions. All of these objects have angular rates that, for at least part of

their orbit, will be within the capability of moving target tracking software being

developed for JWST. Table 1 shows these rates (Anandakrishnan et aI., 2007). Comets

are a special case in which rates vary greatly from object to object, and many comets

exceed JWST's capability.

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Table 1: Tracking rates for various solar system objects

Distance moved Time to move 1 Minimum rate Maximum rate

Object in 10 hrs at min arcmin at max (mas/sec) (mas/sec)

rate (arc sec) rate (hours) Mars 2.5 28.6 90 0.6

Jupiter 0.070 4.5 2.5 3.7

10 0.004 10.2 0.14 1.6

SaturnlTitan 0.040 2.9 1.4 5.7

Uranus 0.020 1.4 0.7 12

Neptune 0.004 1.0 0.14 17

PlutolKBO's 0.160 1.0 5.7 17

By commanding the pointing control system to force the guide star to move opposite the

motion of the solar system target, the Webb will hold the target steady in its IR cameras

or spectrographs (1-28 microns). These observations must finish before the guide star

moves out of the FGS field of view: imposing a limit on the product of the duration and

the apparent angular speed ofthe target. Another important factor in assessing the utility

of JWST is spatial resolution on the surface of a particular solar system body. In table 2

we take the diffraction limit at 2 microns as the criterion for calculating effective spatial

resolution on various solar system objects.

Table 2: JWST spatial resolution for a 2 micron diffraction limit of 0.07 arc sec

Object Diameter. arcsec Diameter, km Spatial

resolution. km

Mars 4-25 6800 66

Jupiter/lo 30-49/.77-1.3 140,000/3600 230

Saturn/Titan 15-20/.65-.87 120,000/5200 420

Uranus 3-4 51,000 840

NeptunelTriton 2/0.11 50,000/2700 1300

Pluto@35AU 0.1 2400 1500

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Finally, JWST will overlap with a number of missions to important outer solar system

and main belt asteroid targets (we do not include Mars missions as these are almost

continuous now). Complementary observations by JWST during and after these missions

will permit extension in wavelength range and time baseline of the mission targets. Table

3 lists these missions, their targets, and anticipated operating periods relevant to JWST.

Table 3: Main belt and outer solar system missions operating during JWST

Qb,jm Mission Operating period

Ceres Dawn 2015

Jupiter Juno 2016-2017

Saturn/Titan Cassini Solstice 2010-2017

Comet 67P/ Rosetta 2014-2015 Churyumov-Gerasimenko Pluto New Horizons 2015

Mars

Continued studies of Mars with JWST offer the promise of unique and important

new contributions to Mars science and to NASA's future mission goals. Specifically,

global-scale near-IR observations can: (1) determine the variability of major and

secondary atmospheric species like C02, CO, and H20, providing data for photochemical

and dynamical modeling of the present Martian climate; (2) constrain the near-IR

radiative and absorptive properties of airborne dust, another key component of the

present Martian climate system; (3) assess the magnitude and scale of diurnal, seasonal,

and interannual volatile transport through direct near-IR detection and discrimination of

surface and atmospheric H20 and C02 ices/clouds, especially in the polar regions; and (4)

help to quantify the surface volatile budget and resource potential by detecting and

mapping the distribution of H20-bearing or OH-bearing surface minerals like clays and

hydrates.

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Mars has a very high surface brightness in the near and mid infrared. NIRCam

can only observe Mars at the longest wavelength, narrow band filter. MIRI cannot

observe Mars in any imaging or spectroscopic configuration. Use of the FGS-

TFI requires the sub-array mode through the neutral density filter. The photometric

calibration of this mode is part of the baseline calibration plan of the FGS team.

Mapping of methane plumes on Mars

Beginning in 2003, ground-based telescopes and orbiting spacecraft detected

methane in the Martian atmosphere (Formisano et al., 2004; Mumma et al. 2009), at

levels as high as 33 parts per billion (Ppb); were this spread uniformly over the planet's

atmosphere this would be equivalent to 6 ppb. By 2006 the equivalent mixing ratio over

the whole planet had dropped by a factor of two. The surprising aspect ofthis discovery,

aside from the potential biological significance of methane itself (either generated by

organisms or if produced geologically, indicative of subsurface water-rock reactions), is

the short decay time of the methane. Although it is unstable in the predominantly COz­

rich atmosphere, the photochemical loss timescale is centuries, and indeed a continuously

emitted source over that time would have led to a uniform mixing ratio of methane

throughout the Martian atmosphere over a much shorter timescale. The methane is

definitely localized in latitude and longitude, and hence what is observed is material that

has been recently emitted with a residence time in the atmosphere significantly less than a

martian year, lost through oxidative reactions with the soil.

While ground-based observations were made with spectral resolutions of 30,000,

for Planetary Fourier Spectrometer aboard Mars Express in orbit around Mars had a

spectral resolution of 3,000 at 3.3 microns, where methane was detected (figure 3). Other

bands of sufficient strength within JWST's wavelength span are at 2.3 microns and 7

microns wavelength, though both are weaker than the 3.3 micron band, and the inability

to use MIRI would rule out the longer wavelength band. While JWST's large distance

from Mars and lack of a very high resolution spectrometer militate against it being a

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useful way to find the ambient background level of methane, it might catch a large

-,...

(cm-1 ) Figure 3. Synthetic spectra designed to simulate data from the Planetary Fourier Spectrometer (PFS) observing the Martian atmosphere from the Mars Express orbiting platform with spectral resolution of 3000. Visible is the methane V3 feature at 3018 cm-I computed for 0 ppb (green curve) and 10, 20, 30, 40, and 50 ppb (violet curves) of methane, compared with the PFS average spectrum (black curve). Results are consistent with between 10-20 ppb ofCH4 in the atmosphere. The synthetic spectra have been computed for 6.7 millibars of C02, including 350 ppm of H20, along with dust and water ice clouds. From Formisano et al. (2004).

outburst if these occur with a frequency higher than once per decade.

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lo's allure is its very unusual nature as the most volcanically active body in the

solar system, and its direct links to both the Jovian magnetosphere (through its associated

plasma torus) and the Jovian atmosphere (via the strong current system that links the

two). Detection of S02 gas absorption at 7.4 11m-seen by Voyager IRIS but not recorded

since-will be of great interest because it records gas abundance directly above hot spots.

This band cannot be observed from the ground due to telluric absorption, and while

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SOFIA can detect the band, it is unable to spatially resolve Io's disk. The mid infrared

spectroscopic capabilities of JWST would also be very useful for exploring subtle 10-20

f.l.m spectroscopic features seen by Voyager IRIS. Furthermore, detection of faint, very

high temperature hot spots at 0.6 - 1.4 f.l.m would be of some importance, as would

detections of neutral S emissions at 1 f.l.m. Roll stability in target tracking and cosmic ray

constraints on the detectors limit exposure times to 1000 seconds; the effect of this on

sensing of hot spots should be evaluated.

Titan

Titan is a target of the very highest astrobiological interest in our own solar

system. It is the second largest moon in the solar system, has a nitrogen atmosphere

several times denser at its surface than that of the Earth at sea level, and boasts an active

equivalent of the Earth's hydrological cycle in which two hydrocarbons, methane and

ethane, take the place ofwater--each cycling on different timescales (Lunine and Lorenz,

2009; Aharonson et a1. 2009).

Titan is richly endowed in organic molecules on the surface; a series of lakes and

seas at polar latitudes hold more hydrocarbon material than the known oil reserves on the

Earth (Lorenz et aI., 2008). Its nitrogen-methane atmosphere, by virtue of continuous loss

of hydrogen liberated from photolyzed methane in the upper atmosphere, is not strongly

reducing, and hence is comparable to the pre-biotic Earth's atmosphere in net redox

propensity for synthesizing organic polymers. While the chemistry of the atmosphere is

well known, that of the surface is not. However, the atmospheric chemistry liberates

hydrogen from methane to create a suite of higher carbon number hydrocarbons and

nitriles, whose fate in the extraordinarily cold atmosphere (94 K at the equatorial surface,

90 K at the poles) must be to condense and fall to the surface. Some of this material falls

directly into the lakes and seas or is transported there by winds; other aerosols

agglomerate to form sand-sized particles comprising the equatorial dunes.

Impacts melt the water ice crust and internally-generated volcanism might as well,

leading to locations on the surface where the organics react with water over significant

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timescales to produce amino and carboxylic acids, as well as other precursors to

biomolecules. Thus, while the low temperatures rule out continuous surface liquid water

and hence life, Titan is a Mercury-sized world upon which some of the steps associated

with the pre-biotic synthesis of polymers essential to life are played out--over and over

again with every impact.

The US-European mission "Cassini-Huygens" entered Saturn orbit in 2004 to

begin an exploration that continues beyond the four-year prime mission into the present

"equinox mission" (2008-2010) (referring to the present seasonal phase of Saturn and

Titan) and the "solstice mission" (2010-2017) after which it will be directed into a

destructive entry of Saturn's atmosphere to avoid contaminating Titan or Enceladus. In

2005 the instrumented Huygens probe descended through the atmosphere to the surface,

relaying images of valley networks evidently carved by liquid methane. In total the

mission has told us that organic deposits are indeed widespread across the surface, and

hints of variations in the organic composition can be seen in the medium-resolution (R~

150 at 2 microns) near-infrared spectroscopy done by Cassini.

However, gaps will remain in our knowledge when final observations of Titan by

Cassini are made in 2017. These gaps are the result of several limitations of Cassini.

First, geometric constraints associated with the fixed-pallet placement of instruments on

the Cassini orbiter (a compromise due to cost) dictate that each of the close flybys will be

devoted to only a subset of the instrument techniques. In the end, radar will cover only

40% of the surface at its best spatial resolutions of hundreds of meters. Second, the VIMS

near-infrared spectrometer has a lower spectral resolution and sensitivity compared to

JWST. The limited spectral resolution is particularly frustrating because Titan's

atmosphere must be viewed through a scattering haze of photochemical aerosols and at

wavelengths in between the deep absorbing atmospheric methane bands. The VIMS

wavelength bands are such that residual methane absorption remains a problem, and

atmospheric models must be used to remove this residuum. Finally, there was no

possibility for Cassini to cover a full Titan year, 29.5 Earth years. Because of the axial tilt

of Titan (essentially coaligned with the spin axis of Saturn), Titan experiences seasonal

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shifts of sunlight similar in amplitude to that of the Earth. Spacecraft missions to date,

and those planned, will cover a portion of Titan's year corresponding to northern late fall

through the fIrst "day" of northern summer (fIgure 4). JWST's period of operation is

unique in that it will cover the portion of Titan's year corresponding to the late northern

spring and almost all of the northern summer.

Figure 4: The seasonal configuration of Titan (identical to that of Saturn) is shown. Designations "Vernal", "Summer" etc. are for Titan's northern hemisphere. The seasonal phases for various spacecraft missions in the past, present and future are shown. JWST's period of operation, assumed here to be ten years from launch in 2014, occupies a unique time during northern summer not probed by other space missions. Note the approximately two-year overlap between Cassini (orange) and JWST (purple) observations. Saturn's orbit around the Sun has an eccentricity of 0.05; "r" indicates the Saturn (Titan) perihelion and aphelion. A proposed Discovery-class mission, Titan Mare Explorer, would arrive in 2023 (not marked on the figure).

HST and adaptive optics ground-based telescopes have achieved diffraction

limited imaging of Titan from Earth. HST NICMOS observations of Titan demonstrated

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spatial resolution of roughly 200-300 km resolution near the Titan equator and sufficient

signal-to-noise to identify the darkest areas as having near-infrared albedos consistent

with hydrocarbons (Meier et aI., 2000). Ground-based telescopic studies can do what

Cassini cannot: provide frequent if not continuous coverage of changes in the atmosphere

and on the surface. Ground-based observations suggested short-term changes in the 1-2

micron region of the spectrum associated with the formation of clouds even prior to

Cassini (Griffith, 2000). In 2008 a major outburst of mid-latitude clouds was observed

from the IRTF (Schaller et aI., 2009).

By 2017 we will be left with the following questions:

1. What does the surface look like in higher (R~3000 vs. 200) near-IR

spectroscopy?

2. What time-variable phenomena might occur due to seasonal (decadal)

variations or stochastic surface events in the near-infrared and in that part of

the mid-infrared (640 cm- I ) where the atmosphere is once again optically thin

enough to see the surface?

JWST can make NIRCam images, and NIRSpec spectra of Titan to build on the

2004-2017 Cassini mission survey, creating a potentially long (10 year +) baseline of

spacebome near-infrared observations of Titan's surface and atmosphere during a

seasonal configuration hitherto unexplored in the infrared. The pixel size on

NIRCamgives about the same spatial resolution on Titan as Hubble (table 2), but the

signal-to-noise is much higher. Spectral resolution a factor of 6 better than on Cassini can

be accomplished using the NIRSpec, allowing for spectra far more diagnostic of the types

of organic species present on the surface. Thus while Cassini gets better spatial

resolution, JWST will achieve higher spectral resolution with useful spatial resolution

over the mid-latitude regions of Titan. Of interest is whether surface changes or secular

atmospheric changes are in evidence over a decadal timescale. With NIRSpec, the ability

to probe the atmosphere over several levels down to the surface provides a unique long­

term capability that is unavailable from Hubble and will cease to be available from

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Cassini after 2017. Thus JWST provides long-time baseline continuity throughout the

infrared.

One approach is to take NIRCam, NIRSpec and MIRI data on Titan over three

equally spaced intervals during the 16-day orbit of Titan, which is phase-locked to

Saturn. This provides images and spectra centered approximately 120 degrees apart from

each other, and hence global coverage. Cloud movement in the stratosphere, based on our

understanding of Titan's winds, occurs with a velocity of 100 mlsec and hence it requires

many hours to track cloud movement in detail. Cloud tracking with HST was very

difficult because of the telescope's 90-minute orbit around the Earth; with JWST at L2 it

will be much easier. Beyond the initial cloud campaign, revisits to Titan once per year

should be done to map in imaging and spectroscopy each of the hemispheres to look for

longer-term changes.

Of additional interest is whether surface changes or secular atmospheric changes

are in evidence over a decadal timescale. The year 2009 corresponds to the onset of

northern hemisphere spring equinox, and the polar regions where large numbers of lakes

and one Caspian-sized sea are present will be experiencing sunlight for the first time in

almost 15 years. Dramatic changes are expected in the atmosphere above the lakes region

and on the surface itself. JWST will be able to monitor these changes beyond the hard

cutoff for detailed observations of Titan by the Cassini mission of2016. Observations

prior to that will allow correlation between Cassini and JWST data, allowing a better

understanding of what the JWST observations mean when Cassini is no longer available.

Should JWST's mission last for a decade, that is, into 2024, it will carry Titan seasonal

observations into northern summer solstice, and connect Cassini with the next potential

mission to Titan, the Titan Mare Explorer, proposed to arrive in 2023.

Giant planets

The giant planets make up most of the mass of the solar system with the exception

of the Sun, and represent natural laboratories for fluid dynamical, including

meteorological processes, distinct from that ofthe Earth and from each other as well. The

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foundational goal to understanding these systems is to quantify how energy, momentum

and gaseous composition are connected/transported between various layers in the

atmosphere (thermosphere, mesosphere, stratosphere, upper and lower troposphere, and

their connection to the charged environment of aurorae/magnetospheres).

The evolution of discrete features such as storms, convective instabilities, waves,

polar vortices and seasonal phenomena should be observed at a variety of wavelengths.

Regular coverage across the spectrum of winds (of jets and vortices) to see how they

change, particularly in response to changes in the thermal and cloud profiles observed in

the IR and visible, is of particular value as well. Center-to-limb studies over multiple

wavelengths and comparison of clouds and hazes from planet-to-planet are important to

understanding formation of clouds and particle size distributions: a particular problem for

modeling the atmospheres of extrasolar giant planets.

Jupiter and Saturn

Jupiter is the archetypical giant planet that provides a "ground truth" for the

hundreds of giant planets seen around other stars. Much of what we know of Jupiter

comes from Earth- ans space-basedbased telescopic observations, including the

spectacular set of results obtained during and after the impact of the Shoemaker-Levy

comet fragments into the Jovian atmosphere. The Galileo mission in the 1990's provided

the first close-up near-infrared spectra of the Jovian atmosphere, revealing a variety of

meteorological phenomenon in the deeper water cloud layer beneath the surface ammonia

clouds, some of which could be tied to optical lightning flashes seen in CCD images from

the Galileo camera. Moist convection-convective activity enhanced and shaped by the

condensation of water and ammonia, may be the dominant process by which energy is

transported vertically through the atmospheres of Jupiter and Saturn. Higher in the

atmosphere, the distributions of hydrocarbons and stratospheric hazes are complex and

serve as tracers of stratospheric dynamics and chemistry.

Juno, a polar orbiting mission that will arrive in 2016 for a one-year mission, will

make near-infrared spectra and images from very close range (within 10,000 km of the

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visible cloud surface), thereby providing extremely high-resolution detail (tens of

kilometers) on convective storm activity. However, the highly elliptical orbit of the

spacecraft means these visits will be brief, and they will lack global context, which JWST

can provide overlapping wavelengths to the Juno infrared mapper via MIRI and possibly

NIRCam (but see below on brightness constraints for JWST constraints regarding bright

objects. ).

Saturn's disk appears bland relative to Jupiter in the optical, but is spectacular in

the near infrared as revealed by the Cassini Visible and Near-Infrared Spectrometer

(figure 5). Beginning in 2016, Cassini will be placed in a highly inclined, highly

eccentric, polar orbit of Saturn that is akin to the orbit of Juno around Jupiter. Although

the intent of this orbit is to eventually destroy Cassini in Saturn's atmosphere (in 2017)

for purposes of planetary protection, these Juno-like orbits will allow high-resolution

near-infrared images of the turbulent Saturn atmosphere revealed to be present below the

haze. As in the Jupiter case, global context will be missing but is important for placing

the detailed images into larger-scale weather systems. JWST will be capable of providing

this information in overlapping wavelength regions via NIRCam and MIRI. A rich variety

of features exist in the infrared diagnostic of various atmospheric processes (figure 6).

These observations, in tum, will help greatly the interpretation of data on exoplanet

weather systems, a newly emergent subfield of extrasolar planetary studies also enhanced

by JWST.

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Figure 5: High spatial resolution thennal imaging of giant planets strips away the haze to reveal dynamical processes within the deeper atmosphere. Saturn seen from Cassini-VIMS in (left) reflected sunlight at 1.6 (green), 2.05 (blue) and 2.79 (red) microns wavelength presents a hazy, relatively featureless surface. In the thennal infrared at 5 microns, Saturn's deeper atmosphere (2 bars) exhibits dynamical processes resembling those seen on Jupiter.

NIRCam can only observe Saturn at the longest wavelength, narrow band filter. NIRCam

cannot observe Jupiter, unless a highly specialized "strip mode" is adopted, which seems

unlikely given that the bright planets are not science drivers for the mission. Jupiter and

Saturn can be imaged in MIRI's neutral density filter, but this filter is not planned for

calibration. The MIRI MRS/IFU can observe Jupiter and Saturn. The TFI can observe

through its neutral density filter.

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Uranus and Neptune

Much less is known about Uranus and Neptune-mass than about Jovian-mass

objects, both within and beyond our solar system. There are important differences, tied to

the proportionately larger effect of a heavy element core in the former relative to the

latter. Neptune had significant influence on the early history, evolution, and development

of the solar system, deflecting icy planetesimals containing water and other volatiles to

inner solar system from the Kuiper Belt. Detailed comparisons of internal structures and

compositions of gas giants with ice giants will yield valuable insights into processes that

formed the solar system.

Uranus and Neptune are each a tenth the mass of Jupiter and Saturn, contain a

higher proportion of heavy elements, and because of their greater distance from the Sun

have different condensables in their atmospheres. The condensable responsible for moist

convection in the upper atmosphere is methane, not water or ammonia, and rapid rotation

plays an even larger role on these objects relative to vertical motions. Observations of

Uranus and Neptune within our own solar system are sparse, and the ability to observe at

a wide range of wavelengths with high sensitivity is of keen value. These two bodies do

not pose saturation problems for JWST.

JWST can image spectral features formed at high altitudes in these atmospheres

with extreme sensitivity. Moving target tracking software will allow exposures in

principle of a day (other operational constraints will require that exposures longer than

that be broken up). This allows mapping of clouds, spectral imaging of specific species

such as H3+ (auroral emission), CO in fluorescence, and others. JWST's unique sensitivity

will allow detailed mapping of the 5-micron window, search for minor species and

measure the isotopic ratios of major elements in trace species.

The mid infrared is observable with JWST both for Uranus and Neptune, up to at

least 22 J.UTI wavelength in many modes. In the mid infrared it is possible to identify

poorly resolved features in both planets (e.g., in the 5-7 micron region, and particularly

deuterated species), track the variability of species with altitude using the R=3000

capability (for example CH4), and determine the spatial distribution and time evolution of

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minor species. Bands of species suspected to be present in these planets but not

confirmed, for example benzene, are detectable with JWST.

Observing modes with JWST instruments exist to observe all the outer planets

without saturating. However, these modes are limited in their science capabilities and, in

some cases, will require adopting new procedures to carry out. NIRCam can observe

Uranus and Neptune using a subarray mode that could be adopted based on a study by

Meixner et al. (2008). While NIRSpec cannot observe all the planets in all its

Figure 6: 1-5 micron spectra of Saturn (top) and Titan (bottom). Phosphine is a good tracer of vertical dynamics. Methane fluorescence provides infonnation on the thermal structure of the upper atmosphere, while methane and CH3D absorption constrain models of giant planet formation. Ammonia is a cloud forming species in Saturn's atmosphere. From Baines et al (2005).

configurations, NIRSpec can do so in the highest spectral resolution and subarray modes

currently planned. Observations of the planets with orbits beyond Earth are possible with

the FGS-TFI. Uranus and Neptune can be observed in imaging mode.

The outer planets represent the brightest targets that may be observed with JWST

and, thus, the most challenging for the bright object limit on the instruments. Table 4

summarizes what can be done for each of the giant planets and Mars, from Meixner et al.

(2008).

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:':-' ;,.pblecf,+ Mars

Jupiter

Saturn

Uranus

Neptune

Comets

Table 4. JWST Bright Object Capabilities

from Meixner et al. (2008)

j2'''A~ \::;~/ ~.\~~IRs~ ;;,'" ~.,:~;.i:·.MI,.:~·iS. ~. '::< . ..~,.' ...... ' ...... -:, :,' ~::;} '::~: ;: '; -

Longest 'A. Highest AI l1'A. No

Strip mode Highest AI l1'A. Neu. dens. tilt and MRS/IFU

Longest 'A. Highest AI l1'A. Neu. dens. tilt and MRS/IFU

Subarray Highest AI l1'A. Spectra and mode imaging

Subarray Highest AI l1'A Spectra and mode imaging

~)i'SW~~~"i< .' .. /':.~~ ... )

TFI

TFI

TFI

TFI

TFI

Comets are remnants of Solar System formation, and their current composition

and physical properties provide a constraint on the conditions in the solar nebula 4.6

billion years ago. Comets were the building blocks of the giant planets' cores. Low

resolution infrared spectroscopy of cometary dust will uncover mineralogical signatures,

which can be compared with those seen in protostellar and planetary debris disks around

nearby young stars and solar analogs, and potentially reveal the isotopic ratios of some

major elements. Many comets fall within the maximum moving target rate studied for

JWST (30 mas/sec). An important example is Comet 67P/Churyumov-Gerasimenko.

The International Rosetta Mission, the Planetary Cornerstone in ESA's long-term

space science program, will rendezvous with Comet 67P/Churyumov-Gerasimenko

(C67P/C-G) in mid 2014, making this object one of great scientific potential for JWST.

Rosetta will study Comet 67P/C-G the nucleus and its environment in great detail for a

period of nearly two years prior to the 2014 rendezvous. After reaching Comet 67P/C-G,

the spacecraft will spend one year mapping and examining the surface using remote

sensing, analyzing dust and vapors, and releasing a 100-kg lander equipped with a drill

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and scientific instruments for in situ analyses. C67P/C-G belongs to the Jupiter family

comets, which represents a large group of the short-period comets (P<20 years) in the

Solar System that are dynamically controlled by the giant planet. Analysis of the orbit of

C67P shows that it is observable by JWST during key portions of the mission.

Observations of comets with JWST will enable investigations of the chemical

composition of cometary ice and dust with unprecedented sensitivity. Near- and mid IR

spectroscopy of cometary comae can be used to measure abundances of H20, CO, C02,

and CH30H in even relatively faint comets. Near-IR spectrometry with R - 1000

resolution will be used to measure the ratio of ortho-to-para H20 (OPR) separately,

possibly providing an indication ofthe comet's formation temperature. Likewise, mid-IR

spectroscopy can determine the mineralogy of cometary dust grains. Finally, JWST's

ability to image cometary nuclei at both mid and near-infrared wavelengths with high

spatial resolution and sensitivity will allow high accuracy measurements of sizes and

albedos of cometary nuclei. The results from cometary programs can be combined with

those from programs investigating circumstellar disks and star formation regions to build

a complete picture of planetary system formation and evolution.

JWST will measure the C02 abundance in comets that come within -3 AU of the

Earth and Sun. Such measurements cannot be done from the ground because of C02 in

the Earth's atmosphere. Depending on the circumstances, JWST can measure C02

emission in either the V3 band near 4.3 JlIIl or the V2 band near 15 J..Lm, both of which are

exceptionally strong. The C02 molecule must be detected from space because strong

absorption in the terrestrial atmosphere prevents ground-based IR observations, and its

lack of a permanent electric dipole moment molecule precludes radio emission.

The deuterium-to-hydrogen ratio (DIH) is an extremely important indicator of the

extent to which Earth's oceans could have been derived from comets. In this regard, the

only three comets for which DIH has been measured are long-period (i.e., Oort cloud

comets; Meier et al. 1995). Short-period comets (i.e, Kuiper Belt comets) which arose

from a reservoir further out in the planet-forming realm than the Oort cloud comets,

might have a different isotopic signature in hydrogen. A lower DIH in short-period

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comets than long-period might permit a larger fraction of Earth's oceans to have been

derived from comets than the 10% figure given by the Oort cloud D/H; a high DIH would

more severely constrain dynamical models by forcing the fractional contribution of

comets to Earth's oceans to be lower. The DIH ratio can be measured in any of the ice

bands, for a variety of hydrogen-bearing molecules. For example, the CH4 V4 band at 7.7

Jim and the CH3D V6 band can be used. For gas components, H2 and HD can be used to

determine the DIH ratio. For example, the S(O) quadrupolar lines at 17.1 Jim can be

observed with JWST.

There are many more issues that can be addressed by cometary spectra. For

example, the spin state-para vs. ortho-ofhydrogen is an important measure of the

extent to which cometary grains were processed during formation, as is the ratio of

amorphous to crystalline ice. The kinds of observations required to make progress on

these abundances and ratios include high-sensitivity infrared spectroscopy at spectral

resolutions exceeding 1000. While ground-based 30-meter telescopes will have very high

sensitivity in the near infrared and can make key contributions there, JWST will be

unparalleled in the region beyond 3 Jim wavelength.

Main Belt Asteroids and Comets

The main belt asteroids are a set of remnants ofthe last stage of planet formation

and, in some cases, the fragments of much larger planetary bodies which differentiated

into iron cores and rocky mantles. They have tracking rates intermediate between those of

Mars and Jupiter and hence are candidates for observation with JWST. Ceres is the one

asteroid large enough (900 km diameter) to be classified as a dwarf planet along with

Pluto and Eris of the Kuiper Belt, and is believed to have differentiated into a rocky core

and an ice-rich mantle, covered in turn by a dusty outermost layer (Zolotov, 2008). It may

also have a tenuous atmosphere (A' Hearn et aI., 1992).

Astronomers have known for more than two centuries that comets can be split

into two groups as defined by their orbits about the Sun. Long-period comets (P>200

years) originate from the Oort Cloud. Jupiter-family comets (P~20 years) originate from

the region beyond Neptune. A new third dynamical class of comets, recognized only in

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2006, orbit much closer to the Sun entirely within the main asteroid belt. These are

referred to as the main belt comets - faint icy asteroids that have dusty eruptions

producing cometary comae. These objects are of particular interest as they could have

been an important source for terrestrial water delivered to the Earth early in its history.

JWST is the only means to characterize the chemical properties (water, hydrocarbons,

mineralogy) of this new class of comets. Their apparent rates of motion are intermediate

between those of Mars and Jupiter.

Kuiper Belt Objects

Kuiper Belt Objects (KBOs) reside beyond the orbit of Neptune (30 AU) and have

apparent rates of motion of <1 to 3 milli-arc seconds (mas) per second relative to the

inertial reference frame (guide stars). The largest of these bodies, including Pluto, are

known as dwarf planets. In addition to these large objects, over 1000 smaller KBOs are

now known (down to d~30 km) and an order of magnitude more are likely to be

discovered over the next five years as large survey telescopes such as PanSTARRS and

LSST begin their observations. JWST will have the ability to obtain R=100 near-IR

spectra and 20-25 J.Ull mid-IR photometry of all known KBOs. The small KBOs are the

remnants of the icy building blocks that formed the giant planets and are analogous to the

material in debris disks that are now being observed around other stars. Understanding

the physical properties of these objects is key for determining the origins and subsequent

evolution of the objects in our solar system and in other planetary systems.

Currently, the only physical property that has been measured for a large number

ofKBOs is object visible color. KBOs have a wide range of colors from neutral (solar­

like) to extremely red. In addition, observations of only the brightest KBOs with Spitzer

have found wide range of albedos from close to zero to in excess of 0.9. The wide range

of optical colors and albedos ofKBOs is likely a result of "nature" (formation location

and compositional differences) and may also include the effects of "nurture" (weathering

processes such as irradiation or collisions that have altered the surfaces over the past 4.5

billion years). This ambiguity may be resolved with near and mid-IR narrow band

23

imaging and spectroscopy of many KBOs with JWST. Unlike visible color, which does

not provide any direct compositional information, near-IR spectroscopy ofKBOs will

record the reflected solar spectrum to identifY absorption features from surface

constituents, such as water ice, hydrocarbons (propane, ethane, etc), water hydration

bands, and nitrile compounds. Mid-IR spectra and imaging ofKBOs longward of ~20

microns will record the intrinsic thermal radiation from the object enabling the

independent determination of its albedo, and hence its diameter. The compositions and

sizes of large numbers of objects will allow us to understand how these properties relate

to dynamical class in the Kuiper Belt. In addition, comparison of these properties with

dynamically related populations such as centaurs (objects with perihelia between the

giant planets) and short period comets will indicate how heating these objects changes

their surface properties.2424

A large fraction ofKBOs are binary which enables the mass of the system to be

determined. Size information from JWST mid-IR photometry combined with the mass

will enable the measurement of a large number of densities, a key physical property

related to composition and porosity. In addition, near-IR spectroscopy of each object

within a binary will determine if the compositions of the objects are indeed alike as is

indicated by their similar visible colors (Benecchi et al 2009). If the surfaces of each pair

within a binary are identical, it indicates that surface composition is likely primordial and

provides a powerful constraint on the initial disk compositional architecture.

Near-IR spectroscopy and mid-IR photometry with JWST will likely reveal

additional information about large-scale collisions in the Kuiper Belt. The Haumea

collisional family was detected because of the unique spectral signature of the family

members (nearly pure crystalline water ice on the surfaces of objects that were close to

each other in orbital element space). Near IR spectroscopy of a large number of objects

spread throughout the Kuiper Belt will help determine the extent of the spread of the

fragments from this large collision (constraining physics of the impact) and may also find

additional clusters of objects with similar surfaces that are fragments of other collisions.

24

Understanding the frequency and the physics of collisions in the outer solar system will

help to determine the initial mass and dynamical excitation of the early Kuiper Belt.

R = 3000 spectroscopy of the largest and closest current and former members of the

Kuiper Belt--Triton and Pluto-- will constrain isotopic ratios in the water ice and other

components, as well as providing surface temperature monitoring through the nitrogen

overtone band. While the near-IR is the most familiar territory in this regard, because of

the ground-based spectra of Triton and Pluto, rotational features will appear in the mid IR

and will provide compositional and isotopic data not attainable in the near-IR overtone

region. Mid IR detection leading to the separation of the size and albedo of such objects

is of fundamental importance, and requires the high mid-IR sensitivity that JWST

provides.

Finally, JWST will allow for the detailed examination of the physical properties of

a new population of objects with orbits that are detached from the Kuiper Belt. While

most KBOs have perihelia within -IS AU of Neptune and are thus dynamically

controlled by it, Sedna, with a perihelion of76 AU, is the harbinger of a new population

of objects in the outer solar system. Sedna's faintness (vmag=21) makes ground-based

near-IR spectroscopy difficult and determination of the ices present on the surface has

hampered by the low signal-to-noise of the spectrum. Sedna's surface is so cold that it

should be able to retain ices too volatile to survive on the surfaces of most KBOs.

Comparisons of the physical properties of Sedna to other KBOs and to debris disks

around other stars will provide insight into the origin and evolution of these populations.

25

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